The present invention relates to a method of removing impurity and an improved electrode capable of removing impurity, and more particularly to a method of removing impurity wherein impurity is ionized with photoelectron caused by ultraviolet ray to have the ionized impurity captured by paired electrodes.
One of the conventional methods for removing impurity by use of photoelectron is disclosed in the Japanese laid-open patent publication No. 5-21410. Electromagnetic waves including an ultraviolet ray are irradiated to a photoelectron emitting material such as a metal, an alloy or a compound having a small photo-electric threshold which corresponds to an energy smaller than the energy of the ultraviolet ray, so as to cause photoelectron to be emitted from the photoelectron emitting material. The emitted photoelectron causes ionization of impurity which is floating in a space in the vicinity of a surface of a substrate. The ionized impurity or charged impurity is attracted to and captured with an electrode having an opposite polarity to a polarity of the ionized or charged impurity for removal of impurity from the space over the substrate.
The above conventional method has the following advantages. First, no movable member is required. There is produced no by-products such as ozone, differently from the cleaning process using ozone. Third, it is possible to remove not only organic impurity but also inorganic impurity.
The above conventional method, however, has the following disadvantages. The ionized impurity is captured with and accumulated onto the capturing electrode. The accumulation of the ionized impurity onto the capturing electrode leads to reduction in capability of capturing the ionized impurity or in efficiency of removal of the impurity. In order to remove the accumulated impurity from the capturing electrode, it is required to stop applying a voltage to the capturing electrode or discontinue removal operations of impurity from the space over the substrate. This discontinuation of applying the voltage to the capturing electrode permits rediffusion or release of the ionized impurity once captured with the electrode into the space over the substrate due to no voltage application to the electrode.
In the above circumstances, it had been required to develop a novel method of removal of impurity and improve a capturing electrode capable of removing impurity with avoiding an excessive accumulation of the captured impurity even without discontinuation of applying the voltage to the capturing electrode.